The present invention relates to a phase detection apparatus for compensating a phase rotation error of a received signal modulated with a quadrature phase-shift keying (QPSK); and, more particularly, to a phase detection apparatus, which is capable of selecting decision boundaries based on a phase noise level and adjusting decision regions.
FIG. 1 illustrates a schematic phase diagram of a signal space for a conventional quadrature phase-shift keying (QPSK).
As shown in FIG. 1, an in-phase component signal (hereinafter, referred to as I-channel signal) and a quadrature component signal (hereinafter, referred to as Q-channel signal) are illustrated. The I-channel signal is generated by multiplying a signal received at a demodulator by a cosine signal with the same frequency as a carrier signal generated by a local oscillator. The Q-channel signal is generated by multiplying the received signal by a sine signal with the same frequency as the carrier signal.
According to QPSK techniques, a received signal at the demodulator is determined based on whether the I- and Q-channel signals are greater than zero or not. However, an original signal with phase xcfx86(I+jQ=exp(jxcfx86)) can be rotated by xcex8 on a communication channel because of a noise signal with a phase xcex8(I+jQ=exp(xe2x88x92jxcex8)). Thus, the demodulator receives a signal with phase error of xcex8(I+jQ=exp(j(txcfx86xe2x88x92xcex8)) and may not determine the signal correctly. Incorrect signal decision at the demodulator increases bit error rate (BER), resulting in performance degradation.
As described, a conventional recovery circuit in the demodulator has high BER because it does not determine correctly the signal received around the decision boundaries due to the phase noise. In FIG. 1, shaded areas are vulnerable to the phase noise.
However, up to date, there is no circuitry for effectively implementing the above problems.
It is, therefore, an object of the present invention to provide a phase detection apparatus, for use in a carrier recovery circuit of demodulator for quadrature phase-shift keying (QPSK), which is capable of adjusting decision regions based on a phase noise level and reducing a bit error rate caused by a phase rotation error.
It is an aspect of the present invention to provide a phase detection apparatus for receiving an I-channel signal an a Q-channel signal as a received signal modulated with a quadrature phase-shift keying and compensating a phase rotation error of the received signal, comprising: a phase detector for detecting a phase error through the use of the I- and Q-channel signals and providing an error signal; a shifting means for shifting said I-channel signal by a plurality of predetermined different numbers of bits and providing shifted I-channel signals; a first multiplexing means, in response to an external selection signal, for selecting signals among said shifted I-channel signals; a subtracting means for providing difference signals between said selected signals and said Q-channel signal; a comparison means for comparing said difference signals with a reference signal and producing logic signals; a logic gate for producing a logically ORed signal of said produced logic signals; and a second multiplexing means, in response to said ORed signal, for selecting a signal from said error signal and said reference signal.